Surge Tank
Theory of Operation
The surge tank is the initial capture point for all water returned from the pool. Water is returned from the drains (at the bottom of the diving well) and from the gutters (around the pool perimeter). Water from the surge tank flows into the strainer basket prior to entering the main pump, as shown in the process flow diagram.
The flow from the gutters is highly variable, determined by the amount of activity in the pool and the pool water level itself. When the pool is very active, waves roll into the gutter drains. A sudden surge in pool activity can cause a surge in gutter flow, hence the name for the tank.
When a high amount of gutter flow is present, the drain flow must be restricted. Otherwise, the surge tank level rises and the gutter drain rate slows. Eventually, the gutters may appear to overflow, as the rate at which the water flows into the surge tank is slower than the rate at which the water is flowing in. This poses two issues:
- The amount of gutter water processed is reduced. The gutters capture water that has the potential to be the dirtiest in the pool. Debris (such as leaves, pollen), algae spores, and oils (from the skin, sunscreen lotion) collect on the surface. The sun's UV rays break down disinfectant (chlorine) most at the surface. It is important that this water be collected with priority.
- The calming effect of the gutters is reduced, as waves are no longer absorbed. This is undesirable during competition, as can will affect swimmers' lap times.
A low amount of gutter flow can also pose an issue. This occurs when the pool water level is low and/or there is little activity in the pool. If the drain flow is not increased accordingly, the surge tank level can drop, as the rate at which the water is pumped out exceeds the rate at which water enters.
Troubleshooting
When the automation system is operating properly, the surge tank is mostly self-sufficient. However, the operator may still observe issues from time to time.
Low Water Level
When the surge tank is too low, this is typically due to one of more of the following reasons:
- The pool water level is too low (the modulating drain valve is not present). Fill the pool.
- The modulating drain valve is malfunctioning, if present. Service the valve and/or controls.
- The main pump flow rate is too high. Reduce the outlet flow rate as detailed here (only after ensuring that other conditions are not present).
When a drain return modulating valve is not present, it is important that the drain return is restricted (the valve is partially closed). Otherwise, flow from the drain will potentially prevent adequate flow from the gutter return. In an ideal case, the surge tank should always have a modulating valve on the drain return.
High Water Level
Typically, high water in the surge tank is not of concern (unless it has nearly reached or exceeded the overflow point, of course). High water is usually the result of:
- A nominal or slightly elevated pool water level in conjunction with a large amount of pool activity (spilling water into the gutters).
- Limited flow into the filter system due to a plugged strainer basket or blockage elsewhere.
- Excess water in the pool due to over-filling or a significant rain event (in this case, some water may be removed via backwash at the operator's discretion).
All three scenarios, of course, assume the main pump is operating. The surge tank will rise to a relatively high level shortly after the main pump is disabled (it will match the pool level).
Known Deficiencies
A combination of age and inadequate repairs in the past have led to a number of known deficiencies with the surge tank. These deficiencies have both functional and (to a much lesser extent) aesthetic implications.
All issues are as identified in January 2022. These issues should be corrected as soon as possible.
- There is no modulating valve on the drain return. The impact of this is described in the theory of operation. According to the pool annual inspection, 80% of the total flow to the surge tank must be from the gutters. It is difficult to maintain this operating status without a modulating valve.
- There was, at some point, a Fisher valve installed. A stainless steel float ball, with piping for hydraulic control, is still present in the tank. It is not known when this was removed and replaced with manual isolation valves.
- The isolation valves between the pool and surge tank are failing or have failed. It appears that in Club History#2016, the pool drain valve failed to the point that it could no longer be turned. In response to this, an additional butterfly valve was installed inside the surge tank to allow the pool to be shut down (all water from the pool must be isolated). This is very risky, as the drain isolation cannot be accessed from outside the surge tank. Watching a service person open or close this valve is an "interesting" event.
- The surge tank drain-to-sump valve has nearly failed. During the pool closing in 2021, the service person had a very difficult time opening this valve. If it breaks, the pool cannot be opened.
- The exit pipe is oddly routed, likely in response to a cheap fix when piping was replaced. The exit pipe should be straight, eliminating a multitude of 90's between the surge tank and the strainer basket.
- The tank leaks at several penetration points. Rudimentary fixes with hydraulic cement have been attempted, but many are failing.
- Multiple old penetrations exist, providing potential leak points. This is also an aesthetic issue.
- The inside surface of the tank has deteriorated and must be stripped and coated for longevity.
- There is no overflow pipe. It was capped-over at some point. This is a minor code issue.
Workarounds
- To address the lack of modulating valve, the drain return valve is set to be only partially opened. By doing so, only a portion of the pump demand is met by the drain return. The rest must be supplied by the gutter return. This works to an extent, but:
- If the pool water level is too low, insufficient gutter flow can lead to a critically low level in the surge tank. At this point, the automation system will shut down the main pump for safety. This is typically mitigated by engaging the auto-fill (which also feeds into the surge tank).
- When the pool is very active, there is no way to reduce drain return flow to increase gutter return flow. Hence, the gutters are prone to overflow during busy periods.
- Some additional patching has been applied to seal leaks, but it has shown to be ineffective. As of 2021, small but persistent leaks were observed.
Future Overhaul
To fully address the known deficiencies, an overhaul of the surge tank is planned as the budget permits.
Isolation Valves
There are three isolation valves to be replaced. Details to follow.
Maintenance Drain Valves
There are three maintenance drain valves - not to be confused with the pool drain return valve(s). Details to follow.
Modulating Drain Valve
The modulating valve utilizes a motorized actuator to turn a butterfly valve on the drain return. The term "modulating" implies that the valve can be fully open, fully closed, or operating at a point in-between.
Motorized Actuator
A Bray 70-0201H motorized valve has been selected for the application. This is a Bray Series 70 Actuator, with a 2,000 in-lb turning capacity, configured for 120 VAC on/off operation. A heater and auxiliary contacts are included.
The part number breakdown is approximately as follows (some interpretation):
| Part Number | Description | Comment | |
|---|---|---|---|
| Series | 70 | Series 70 (On/Off) | |
| Supply | (blank) | 120 VAC | |
| Torque | 0201 | 2,000 lb-in torque | |
| Heater | H | Heater is included | |
This particular model has:
- A single-phase reversible permanent split capacitor induction motor.
- 2 x SPDT 10 A auxiliary switches.
- 2 x 3/4" NPT conduit entries.
- A 5 W PTC style heater (to drive out condensation).
- 30 second stroke time (at 60 Hz).
Other notes:
- The motor has an internal thermal trip set at 135 °C.
- Terminals are sized for #22 AWG to #14 AWG.
This actuator has been purchased as of January 2022 and is awaiting installation.
Previously, a Bray 70-24-0201SVH-BBU was recommended. However, 24V operation, the internal 4-20 mA controller, and the battery backup unit (BBU) all add unnecessary cost.
Valve
The recommended valve is a Bray 300800-11010119. An exact breakdown of the part number cannot be located, but it appears that it means the following:
| Part Number | Description | Comment | |
|---|---|---|---|
| Series | 30 | Series 30 (wafer) | |
| Size | 0800 | 8 inch | |
| Base Part | 11010 | TBD | Unable to locate specifics. |
| Material Code | 119 |
| |
This actuator has been purchased as of January 2022 and is awaiting installation.
Piping
Details to follow.
Structural and Surface
Details to follow.